Lossless 10-bit colorspace (yuvj420p) video output
You can't.

When working on footage, it is always most prudent to operate on the frame level, with controlled decoding to the file format you choose. Codecs always have been and always will be an awful choice.

Currently in Blender, all codecs via the VSE (and otherwise I believe) are decoded using the FFMPEG call that converts them to an 8 bit buffer.

So, decode outside to a suitable 16 bit container such as TIFF, then properly linearize the results via a controlled transform.

Also, “8 bit” is not a colour space, nor is “10 bit”, nor is “float color space”.